You can't. A new boundary is always created when sending a message.
Indy does not currently preserve existing boundaries.

> Or how have I to set Boundary for problematic TIdText?

You can't. And it is not just a TIdText that is affected. The new
boundary is applied at the TIdMessage level. There is no workaround
to this issue, other than to simply not use TIdMessage for the sending
in the first place. You already have your needed data in a stream.
Send it to the server manually. That way Indy doesn't alter the data.